Diffusion pumps include a pool of oil that is vaporized in a boiler usually by a high watt density (e.g., 30 watts/in.sup.2) electric heater. One prior art diffusion pump boiler includes a tubular electric heater which has been swaged into a groove in a steel platen. Heat is transferred from the tubular heater to the platen and to a flat contacting side of a metal plate which constitutes the boiler plate of the diffusion pump. Heat propagates through the platen to a flat face of a heavy metal boiler plate and then to the opposing face that forms a floor for the pool so that heat is transferred directly from the plate to the pool.
The electric heater typically comprises a metal tubular sheath, usually formed of Inconel, that surrounds a spiral filament, formed, usually of Nichrome. A space between the filament and the interior surface of the sheath is filled with a material which is both an electrical insulator and a thermal conductor, e.g., magnesium oxide. Terminals on each end of the sheath are usually threaded nickel rods welded to the ends of the filament and brought out past the ends of the sheath so that lead wires may be connected to them. These tubular heaters are easily formed into various configurations, such as circles, helices and spirals. The high watt density tubular heaters of diffusion pumps must be thermally loaded to such an extent that the heat loss from the sheath keeps all exterior sheath portions below 1500.degree. F. Sheath temperatures in excess of 1500.degree. F. cause rapid deterioration and, hence, short life of the heater.
In order to provide the heater with sufficient thermal loading to prevent overheating, a substantial portion of the sheath must be in contact with the boiler plate. This is not as simple as it might appear. A typical heater shape might be a 120.degree. sector. The configuration of the tubular heating element within the boundaries of the sector is designed in such a way that it has a maximum length. In this way the watt density is kept as low as possible. It would seem that if the heater were sandwiched tightly between the boiler plate and a flat heavy clamping plate, that the side of the entire length of the heating element facing the boiler plate would be in contact with it. I have found this is not the case. The diameter of the heater sheath may have gradual variations along its length of .+-.0.015 inches from the mean diameter. As a result, the heater sheath makes firm contact with the boiler plate only where the diameter is at a maximum. In the absence of firm contact with the boiler plate, the transfer of heat must be accomplished by convection and radiation which is much less efficient than conduction. The temperature of the heater may become excessive at the regions where there is no contact, regions referred to as hot spots.
One structure which has been successfully employed to prevent hot spots along the length of a tubular diffusion pump electric heater is disclosed in U.S. Pat. Nos. 3,168,775 and 3,275,801. The structure of these patents includes a cast platen containing a groove into which the tubular heater is forced. The sheath of the tubular heated is then struck with a hammering tool so that the sheath abuts against the surface of the groove. The face of the platen in contact with the boiler plate is ground flat. The platen is bolted to the boiler plate. While this structure has been successful in preventing the formation of hot spots, it is relatively expensive to manufacture and the manufacturing process can result in damage to the tubular heater if the hammer blows are excessive.
It is, accordingly, an object of the present invention to provide a new and improved electric heater.
Another object of the invention is to provide a new and improved electric heater particularly adapted for diffusion pumps.
An additional object of the invention is to provide a new and improved, relatively inexpensive, electric heater having a tubular electric heater with a sheath that is loaded sufficiently so the tendency to develop hot spots is obviated, whereby the heater is susceptible to long life.
A further object of the invention is to provide a new and improved electric heater including a tubular electric heater having a sheath that is susceptible to variations in geometry but which is loaded sufficiently by an inexpensive structure to prevent hot spots from developing.